Metabotropic GABAB receptors mediate inhibitory neurotransmission in the mammalian brain and are central to its function. GABAB receptors are G-protein coupled receptors (GPCRs) that regulate the activity of Ca2+ and K+ channels, and inhibit the function of adenylyl cyclase. Malfunction of GABAB receptors has been implicated in the development of a number of diseases including spasticity, epilepsy and pain. Therefore, elucidating the structure and function of GABAB receptors would assist the design of valuable therapeutic agents. Unlike most GPCRs, GABAB receptors function as a heterodimeric assembly of GBR1 and GBR2 subunits. Heterodimerization is required for the trans-activation of these receptors, where GBR1 is responsible for ligand-binding and GBR2 is involved in G-protein coupling. The first part of this proposal is aimed at understanding the role of GBR1 and GBR2 ectodomains in the activation process. We propose to solve the structures of the individual subunits and study their interactions using biochemical methods. The second aim is to reveal the structural basis of ligand-binding and dimerization of an ectodomain heterodimer, and determine the affinity and specificity of receptor-ligand interactions. Our third aim is to understand the molecular basis of heterodimerization by the intracellular coiled-coil domains and their role in receptor assembly and trafficking. The last part of the proposed research is focused on understanding transmembrane signal transduction mechanism using full-length receptors. We propose to carry out functional studies of wild-type and mutant receptors using whole cell patch clamp methodology. Mutations that affect ligand binding, receptor heterodimerization, and receptor activation will be designed based on the available structural information and homology models. A combination of structural and functional analysis of GABAB receptors will advance our understanding of the molecular basis of GABA action and the activation mechanisms particular to GPCR heterodimers.